【正文】 the gear blank so that all its tooth spaces are simultaneously cut. The radial position of the blades is governed by two interacting conical rings which contact the guide faces of the blade tongs, as may be seen in the sectional view of the cutter head in Fig. every working stroke, the conical rings make a small upward movement, so that the outer shell of the inner conical ring allows the blades to retract from the workpiece during the return stroke. Before each new working stroke, the conical rings move with an additional infeed motion in a downward direction, so that the inner shell of the outer conical ring causes the clearance distance to be cancelled out. In addition, this moves all the blades further towards the workpiece by the desired depth of feed. Figure shows the working area of such a broaching machine with the chuck, gear blank and cutter head. Fig. External broaching of involute gear teeth using the ShearSpeed techniqueThe technique is only economic for largequantity production, as a new cutter head is required for each blank diameter. In special cases, it is possible to produce internal gear wheels with this method. When tube broaching, the work, as in the case of the ShearSpeed technique, is mounted on a mandrel and pushed upwards into the tool tube of a hollow broach. Within the tubebroaching tool, radial cutting inserts are arranged with progressive height increases and fixed. Between the inserts, guides are provided to control the workpiece mounting head during the broaching process. Bevel gears Bevelgearbroaching machines usually work in accordance with the 39。, occurs continuously in the diagonalhobbing method, as here the feed consists of an axial and tangential ponent. The leading dimension s and setting values of a hob are shown in Fig. in which the hob is shown in the actual cutting position. The angular setting (angle of incidence, η) of the hob is dependent upon the direction and value of the helix angle β of the tooth to be cut (if any) andthe helix angle γ of the worm on the hob. With any one hob, gears of any given number of teeth and helical tooth angle, as well as all kinds of tooth profile modifications, may be produced by differing machine settings, provided the teeth are of the same module and pressure angle. Any limitation is only as a result of the working capacity of the machine. A simplified layout of the drive of a hobbing machine is shown in Fig. . The main motor drives the hob directly and also the work table through the pickoff change gear train and the intermediate telescopic worm and worm wheel drive. The choice of ratio in the pickoff change gear train coordinates the rotations of the tool and work, which are dependent upon the number of teeth to be cut on the gear and the number of start s of the helix on the hob. The worm drive drives the feedchange gear train or, as shown in Fig. , the rotation of the axial feed spindle through a stepless variable drive. For the manufacture of helical teeth and diagonal hobbing, the gear blank to be cut is subjected to an additional rotary movement relative to the hob feed, provided by the differential drive. The differential cage is released and set into motion by the differentialchange gear train, having selected the appropriate gears. A detailed representation of the kinematics of such a hobbing machine is shown in Fig. , which also shows how the rotary motion for the radial feed spindle is obtained. Figure shows a general picture of a hobbing machine. On conventional machines, the column is bolted on to the machine bed. The workspindle slide and its support centre are driven by the radialfeed spindle and moved horizontally to obtain a radial feed. The hob spindle can be moved along the hobbing slide through a tangential drive and a tangential spindle, and may also be angularly adjusted to acmodate the setting of the required helix angle for the gear to be cut. The hob drive and the feed drive are located in the column. On a machine of recent design, the table is stationary and the column slides along the machine bed. Only the main drive shaft and the feed shaft are in the column. All other drive elements are in a separate drive box on the left side of the machine in order to improve heat dissipation. In order to provide backlashfree drives, modem machines have all feed shafts fitted with preloaded, recirculating ball nuts. The table drive may be made backlashfree either by the employment of two axially opposed prestressed worms or by the use of a so called 39。 crown wheel. The gear blank makes a generating motion against the rolling plane of the crown wheel, from which the tooth profile develops. The possible movements of such an inserted toothcutter head bevelgearhobbing machine is shown in Fig. . The generating motion during the process is executed about the axes of the gene rating drum (rocker) (2) and the gear blank (1)。 the resulting small chip volumes introduce initial rubbing or pression and may cause a resistance at the beginning of the cut. In the ease of radialaxial hobbing, the hob is first fed radially into the gear blank to the desired tooth depth, and then hobbed using either upcut or downcut motions. After a given number of tooth spaces or gear blanks have been machined, the hob is moved tangentially by a given amount in order to use all the teeth of the hob equally. This action, sometimes referred to as 39。 the ShearSpeed technique and the tubebroaching technique. In the ShearSpeed technique the workpiece is pushed upwards through a fixed cutter head (broaching movement). Inside the cutter head (which is illustrated in Fig. , with a single cutting blade), the profiled steel 39。 manufacture of large gear wheels, or in the mass production of very small gear wheels for the precision engineering